We have extended our mechanistic studies of virus-receptor interactions, with focus on two specific enveloped viruses: 1) Hepatitis C virus (HCV). Using specialized mammalian producer cell line carrying a West Nile virus replicon, we devised a novel system to generate HCV virus-like particles in which all viral-encoded components (proteins and RNA) are derived from HCV. We have also used this system to produce infectious wild type HCV of virtually any genotype (confirmed for 1a, 1b, 2b). We have shown that the system can generate infectious particles carrying full-length HCV genomes that replicate in HCV-permissive target cells as well as human liver slices. Evidence suggests that the ability of the WNV-replicon cell line to secrete infectious HCV particles is due not to ongoing function of the WNV replicon, but rather to intracellular changes (presumably including membrane rearrangements) induced by the replicon. The system can also be used to generate HCV particles with specific mutations of interest, to study their effects on the HCV replication cycle. Variations of this system have been developed to measure entry of HCV VLPs wherein the introduction of a single RNA molecule from an infecting particle generates a robust reporter gene signal. The system has the advantage that the signal is dependent only on HCV entry, and not on downstream steps in the virus replication cycle;thus entry can be studied in various cell types, whether or not they can support downstream HCV replication steps. 2) Kaposi's sarcoma-associated herpesvirus (KSHV, human herpesvirus 8). we have begun to apply our studies to a recently described system wherein KSHV infection of primary B cells from human tonsils can be infected with KSHV, including reporter viruses encoding EGFP and RFP to monitor latent and lytic stages, respectively. Our goals are to analyze KSHV entry mechanisms in these primary B cells (low pH entry, direct plasma membrane fusion) as well as to determine the relevant cellular receptors, including assessment of the roles of the various previously implicated receptors (integrins, DC-SIGN, xCT).